Transcript cis185-ROUTE-lecture6-BGP-Part2

CIS 185 CCNP ROUTE
Ch. 6 Border Gateway Protocol Solution
for ISP Connectivity – Part 2
Rick Graziani
Cabrillo College
[email protected]
Last Updated: Fall 2010
Materials
 Show configuration for not being a transit AS
2
Materials
 Book:
 Implementing Cisco IP Routing
(ROUTE) Foundation Learning
Guide: Foundation learning for the
ROUTE 642-902 Exam
 By Diane Teare
 Book
 ISBN-10: 1-58705-882-0
 ISBN-13: 978-1-58705-882-0
 eBook
 ISBN-10: 0-13-255033-4
 ISBN-13: 978-0-13-255033-8
3
L2
PAIX
Customer
Prefixes
Public IP
Address
Bi-lateral
Settlement
Free Peer
Non-transit.
You can get to
our prefixes and
we can get to
your prefixes.
No charge.
Level 3
Transit
Buy transit per Mb/s per
month for routes not know
from non-transit AS’s
EBGP
CENIC (non-transit AS)
Cabrillo
Google
Redistribute
static
MED
AS and
Public IP
Address
LocPref
SVL-Agg1
SAC
EBGP
UCSC
Hurricane Elec
Fully-meshed IBGP
EBGP
AS and
Public IP
Address
SVL
Commodity
Peer
Commodity
Peer
UCLA
LAX-Agg1
LAX
RVR
Comcast
IGP: IS-IS used for next-hop reachability
Public IP
Address
Commodity
Peer
EBGP
Cuesta
Non-transit. You can get to our prefixes and
we can get to your prefixes. No charge.
Internet 2
Research
Peer
NLR
CUDI
Cogent
Research
Peer
Transit
Buy transit per Mb/s
per month for routes
not know from nontransit AS’s
Note: Non-customer prefixes (networks connecting peers) are not included in EBGP updates to peers.
IGP is not redistributed into IS-IS and IS-IS is not redistributed into BGP.
Note: This diagram is by no means accurate but only a very simplified representation of an AS.
4
5
www.cenic.org
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Path Attributes
 Each route has its own set of defined attributes, which can include:
 Path information
 Route preference
 Next-hop
 Aggregation information
 Administrators use these values to enforce routing policy.
 Based on attribute values, you can configure BGP to:
 filter routing information
 prefer certain paths
 customize its behavior
 Every UPDATE message has a variable-length sequence of path attributes.
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Path Attributes
 Not all vendor implementations of BGP recognize the same attributes.
 Path attributes come in four different types:
 Well-known mandatory
 Well-known discretionary
 Optional transitive
 Optional non-transitive
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Path Attributes
Well-known mandatory
 An attribute that has to exist in the BGP UPDATE packet.
 It must be recognized by all BGP implementations.
 If a well-known attribute is missing, a notification error will be generated
 This ensures that all BGP implementations agree on a standard set of
attributes.
Example: AS_PATH attribute
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Path Attributes
Well-known discretionary
 An attribute that is recognized by all BGP implementations
 But may or may not be sent in the BGP UPDATE message.
Example: LOCAL_PREF
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Path Attributes
Optional transitive
 An attribute that may or may not be, recognized by all BGP
implementations (thus, optional).
 Because the attribute is transitive:
 BGP should accept and advertise the attribute even if it isn’t
recognized.
Example: COMMUNITY
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Path Attributes
Optional non-transitive
 An attribute that may or may not be, recognized by all BGP
implementations.
 Whether or not the receiving BGP router recognizes the attribute, it is nontransitive:
 Should NOT be passed along to other BGP peers.
Example: MED
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WLam Decision Process (Cisco)
The Route Selection
Summary of the BGPWeight
Path Selection Process
 BGP uses the following
criteria,
in the order presented, to select a path for a
Local
Preference
destination:
as path
 NOTE: Not all of these
are commonly used and will be examined in more
detail later in this presentation and in the next presentation.
med
1. If the path specifies a next hop that is inaccessible, drop the update.
2. Prefer the path with the largest weight.
3. If the weights are the same, prefer the path with the largest local preference.
4. If the local preferences are the same, prefer the path that was originated by
BGP running on this router.
5. If no route was originated, prefer the route that has the shortest AS_path.
6. If all paths have the same AS_path length, prefer the path with the lowest
origin type (where IGP is lower than EGP, and EGP is lower than
Incomplete).
7. If the origin codes are the same, prefer the path with the lowest MED
attribute.
8. If the paths have the same MED, prefer the external path over the internal
path.
9. If the paths are still the same, prefer the path through the closest IGP
neighbor.
10. Prefer the path with the lowest IP address, as specified by the BGP router ID
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The AS-Path
Attribute
WLam
Weight
Local Preference
My path to
192.168.1.0 is
(65500, 64520)
as path
med
 AS-path attribute – Well-known mandatory attribute
 Whenever a route update passes through an AS, the AS number is prepended to
that update
 AS number is put at the beginning of the list when it is advertised to the next
EBGP neighbor.
 Router A: advertises network 192.168.1.0 in AS 64520.
 Router C: prepends its own AS number to it and advertises the route to Router B.
 Router B: From Router B’s perspective, the path to reach 192.168.1.0 is:
 65500, 64520
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The AS-Path
Attribute
My path to
192.168.1.0 is
(64520)
My path to
192.168.1.0 is
(65500, 64520)
 AS numbers are prepended only by routers advertising routes to EBGP
neighbors.
 Routers advertising routes to IBGP neighbors do not change the AS-path
attribute
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 show ip bpg – Shows the BGP Table
 AS Path to 44.0.0.0 is via AS: 7018, 22822, 22822, 2152, 7377
 2282 twice?
 AS Path prepend (later)
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The Next-Hop
Attribute
 Next-hop attribute - A well-known mandatory attribute
 Indicates the next-hop IP address that is to be used to reach a destination.
 IGPs:
 hop-by-hop routing protocol
 router-by-router
 BGP:
 hop-by-hop routing protocol (like IGPs)
 AS-by-AS (not like IGPs)
 By default, the next-hop is the next AS
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The Next-Hop
Attribute
 EBGP: The next-hop is the IP address of the neighbor that sent the update.
 Router A advertises (EBGP) 172.16.0.0 to Router B, with a next hop of
10.10.10.3
 IBGP: The next-hop is carried into IBGP unchanged.
 Router B advertises (IBGP) 172.16.0.0 to Router C, with a next hop of
10.10.10.3.
 Router B uses 10.10.10.3 as the next-hop attribute to get to 172.16.0.0
 Router C’s next hop to reach 172.16.0.0 is 10.10.10.3 not 172.20.10.1
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The Next-Hop
Attribute
 Very important that Router C knows how to reach the 10.10.10.3 (10.0.0.0
or 10.10.10.0)
 IGP
 static route
 Otherwise, Router C will drop packets destined for 172.16.0.0, because it
will not be able to get to the next-hop address for that network.
 Must either:
 Advertise the next-hop network, the network between AS’s, (10.10.10.0)
into the AS
 Change the next-hop address (Router B’s 172.20.10.0 network) - later
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The Next-Hop
Attribute
Recursive Lookup
 IBGP router performs a recursive lookup to find out how to reach the BGP nexthop address by using its IGP entries in the routing table.
 Router C has a packet to send to 172.16.100.1
 Finds the longest-match for 172.16.100.1 (172.16.0.0/16) in the routing table
and finds a BGP route with a next hop of 10.10.10.3.
 Does a recursive lookup in the routing table for a path to network 10.10.10.3
 If there is an IGP route to 10.10.10.3 (ex: 10.10.10.0) in the routing table with
a next hop of 172.20.10.1
 Forwards the packet destined for 172.16.100.1 to 172.20.10.1
 Otherwise, drops the packet
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Third-Party NextHop
I’m your EBGP peer but
your best next-hop path
to 172.30.0.0 is via
Router C at 10.10.10.2
172.30.0.0
EBGP
Third-party Next-Hop:
 When running BGP over a multiaccess network such as Ethernet, a BGP
router uses the appropriate address as the next-hop address (by changing
the next-hop attribute) to avoid inserting additional hops into the path.
 Because the network among the three routers A, B, and C is a multiaccess
network, it makes more sense for Router A to use Router C as a next hop to
reach 172.30.0.0, rather than making an extra hop via Router B.
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Third-Party NextI’m your EBGP peer but
Hop
since this is a
multicaccess network I
know your best next-hop
path to 172.30.0.0 is via
Router C at 10.10.10.2
172.30.0.0
EBGP
PVC
PVC
Thanks but I don’t have
a PVC or a Frame Relay
map entry to 10.10.10.2
 However, if the common medium between routers is a nonbroadcast
multiaccess (NBMA) medium, complications might occur.
 Routers A, B, and C are connected by Frame Relay.
 Router B can reach network 172.30.0.0 via 10.10.10.2 (PVC).
 When Router B sends a BGP update to Router A about 172.30.0.0, it uses
10.10.10.2 as the next hop, not its own IP address (10.10.10.1).
 A problem arises if Routers A and C do not know how to communicate directly
 Routers A and C do not have a Frame Relay map entry to reach each other
 Solution: Router B could advertise itself as the next-hop address for routes sent22
to Router A like a non-multiaccess network (later)
The Origin
Attribute
 The origin attribute is a well-known mandatory attribute
 Defines the origin of the path information.
 The origin attribute can be one of three values:
 IGP
 The route is interior to the originating AS.
 Normally when the network command is used (coming)
 Indicated with an “i” in the BGP table (coming)
 EGP
 The route is learned via EGP
 EGP is legacy and no longer supported in the Internet
 Indicated with an “e” in the BGP table.
 Incomplete
 The route’s origin is unknown or is learned via some other means.
 This usually occurs when a route is redistributed into BGP.
 Indicated with a “?” in the BGP table.
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The Local
Preference
Attribute
WLam
172.16.0.0
IBGP
Weight
Local Preference
as path
med
 Local preference - A well-known discretionary attribute
 Indicates to routers in the AS which path is preferred to exit the AS
 Path with the higher local preference is preferred.
 Configured on a router
 Exchanged only among routers within the same AS
 Passed only via IBGP not via EBGP
 Default value on a Cisco router is 100
 Local Preference takes precedence over AS_PATH
 WLam: Weight, Local preference, as path, med
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The Local
Preference
Attribute
172.16.0.0
My Local Preference
is higher so I am the
preferred exit point.
IBGP
 AS 64520 receives updates about network 172.16.0.0 from two directions:
 via AS 65500 (65500, 65350)
 via 65000 (65000, 65250, 65350)
 Router A and Router B are IBGP neighbors.
 Local preference:
 On Router A for network 172.16.0.0 is set to 200
 On Router B for network 172.16.0.0 is set to 150
 Local preference information is exchanged within AS 64520 via IBGP
 All traffic in AS 64520 addressed to network 172.16.0.0 is sent to Router A
as an exit point from AS 64520.
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The MED
Attribute
172.20.0.0
WLam
Weight
Local Preference
as path
med
 The MED attribute - An optional nontransitive attribute.
 Also called the metric
 Displayed in the metric column in the BGP table.
 Indicates to external neighbors the preferred path into an autonomous
system.
 A way for an AS to try to influence another AS as to which way it should
send its traffic when there are multiple entry points.
 MED is sent to EBGP peers:
 Those routers propagate the MED within their AS
 But do not pass it on to the next AS
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172.20.0.0
The MED
Attribute
My MED is 150 for
172.20.0.0 so send
those packets this
way.
My MED is 200 for
172.20.0.0 so send
those packets this
way.
Which path is most
attractive?
 By default, a router compares the MED attribute only for paths from
neighbors in the same AS.
 By using the MED attribute, BGP is the only protocol that can affect
how routes are sent into an AS.
 Router B has set the MED attribute to 150 for 172.20.0.0
 Router C has set the MED attribute to 200 for 172.20.0.0
 Router A receives EBGP updates from Routers B and C
 Chooses Router B as the best next hop to get to AS 65500
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The MED
Attribute
AS
65001
My MED is 150 for
172.20.0.0 so send
those packets this
way to AS 65001.
AS 65000
172.20.0.0
AS
65002
My MED is 200 for
172.20.0.0 so send
those packets this
way to AS 65002.
Different AS’s so I won’t
use the MED unless
configured with “bgp
always compare med”
 By default, a router compares the MED attribute only for paths from
neighbors in the same AS.
 If these were two different AS’s then Router A would use MED for best path
selection
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The Weight
Attribute
(Cisco Only)
WLam
Weight
Local Preference
as path
med
 Weight attribute - a Cisco-defined attribute used for the path-selection
process.
 Configured locally and not propagated to any other routers
 Higher weight is preferred when there are multiple routes to the same
network.
 Weight takes precedence over Local Preference
 Value from 0 to 65535.
 Default of 32768
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The Weight
Attribute
(Cisco Only)
AS 65000
200
AS 65500
150
Router A
 Router A has two ways to reach 172.20.0.0
 via Router B (AS 65000)
 via Router C (AS 65500)
 Router A is configured to set the weight of updates coming from:
 Router B to 200
 Router C to 150
 Weight for Router B is higher so Router A uses Router B as a next hop to
reach 172.20.0.0
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The Path-Selection Decision Process with a
Multihomed Connection
 An AS rarely implements BGP with only one EBGP connection, so generally
multiple paths exist for each network in the BGP forwarding database.
 Using the 11-step route selection process as outlined in Diane Teare’s
book…
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The Route Selection Decision Process (Cisco)
Summary of the BGP Path Selection Process
 BGP uses the following criteria, in the order presented, to select a path for a
destination:
 NOTE: Not all of these are commonly used and will be examined in more
detail later in this presentation and in the next presentation.
WLam
1. If the path specifies a next hop that is inaccessible, drop the update.
Weight
2. Prefer the path with the largest weight.
Preference
3. If the weights areLocal
the same,
prefer the path with the largest local preference.
4. If the local preferences
as pathare the same, prefer the path that was originated by
BGP running on this router.
med
5. If no route was originated,
prefer the route that has the shortest AS_path.
6. If all paths have the same AS_path length, prefer the path with the lowest
origin type (where IGP is lower than EGP, and EGP is lower than
Incomplete).
7. If the origin codes are the same, prefer the path with the lowest MED
attribute.
8. If the paths have the same MED, prefer the external path over the internal
path.
9. If the paths are still the same, prefer the path through the closest IGP
neighbor.
10. Prefer the path with the lowest IP address, as specified by the BGP router ID
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The Route Selection Decision Process (ROUTE)
 Step 1 Prefer the route with the highest weight. (Recall that the weight is Cisco-proprietary
and is local to the router only.)
 Step 2 If multiple routes have the same weight, prefer the route with the highest local
preference. (Recall that the local preference is used within an AS.)
 Step 3 If multiple routes have the same local preference, prefer the route that was originated
by the local router. (network command)
 Step 4 If none of the routes were originated by the local router, prefer the route with the
shortest AS-path.
 Step 5 If the AS-path length is the same, prefer the lowest origin code (IGP < EGP <
incomplete).
 Step 6 If all origin codes are the same, prefer the path with the lowest MED. (Recall that the
MED is exchanged between autonomous systems.) The MED comparison is done only if the
neighboring AS is the same for all routes considered, unless the bgp always-compare-med
router configuration command is enabled.
 Step 7 If the routes have the same MED, prefer external paths (EBGP) over internal paths
(IBGP).
 Step 8 If synchronization is disabled and only internal paths remain, prefer the path through
the closest IGP neighbor. This means that the router prefers the shortest internal path
within the AS to reach the destination (the shortest path to the BGP next-hop).
 Step 9 For EBGP paths, select the oldest route, to minimize the effect of routes going up
and down (flapping).
 Step 10 Prefer the route with the lowest neighbor BGP router ID value.
 Step 11 If the BGP router IDs are the same, prefer the route with the lowest neighbor IP
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address.
The Route Selection Decision Process (ROUTE)











Step 1 Prefer the route with the highest weight. (Recall
that the weight is Cisco-proprietary and is local to the
router only.)
Step 2 If multiple routes have the same weight, prefer the
route with the highest local preference. (Recall that the
local preference is used within an AS.)
Step 3 If multiple routes have the same local preference,
prefer the route that was originated by the local router.
(network command)
Step 4 If none of the routes were originated by the local
router, prefer the route with the shortest AS-path.
Step 5 If the AS-path length is the same, prefer the
lowest origin code (IGP < EGP < incomplete).
Step 6 If all origin codes are the same, prefer the path
with the lowest MED. (Recall that the MED is exchanged
between autonomous systems.) The MED comparison is
done only if the neighboring AS is the same for all routes
considered, unless the bgp always-compare-med router
configuration command is enabled.
Step 7 If the routes have the same MED, prefer external
paths (EBGP) over internal paths (IBGP).
Step 8 If synchronization is disabled and only internal
paths remain, prefer the path through the closest IGP
neighbor. This means that the router prefers the shortest
internal path within the AS to reach the destination (the
shortest path to the BGP next-hop).
Step 9 For EBGP paths, select the oldest route, to
minimize the effect of routes going up and down
(flapping).
Step 10 Prefer the route with the lowest neighbor BGP
router ID value.
Step 11 If the BGP router IDs are the same, prefer the
route with the lowest neighbor IP address.
1
2
3
5
4
7
6
8
9
10
11
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R1(config)#router bgp 65201
R1(config-router)# maximum-paths 2
Multiple Path Selection (BGP Multipath)
 BGP chooses only a single best path for each destination.
 The maximum-paths paths affects only the number of routes kept in the IP
routing table, not the number of paths selected as best by BGP.
 Defaults to one.
 R2 and R3 are advertising network 10.0.0.0.
 With maximum-paths 2: both paths appear in the IP routing table
 Without maximum-paths 2: only one path to 10.0.0.0 in R1’s routing
table
 Only one path is still selected as the best in the BGP table “>” (offered
to the IP routing table) and advertised to its BGP neighbors.
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Configuring BGP
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Entering BGP Configuration Mode
RTA(config)#router bgp 100
RTA(config-router)#
RTB(config)#router bgp 200
RTB(config-router)#
 Use the router bgp autonomous-system global configuration command
 autonomous-system identifies the local AS.
 AS determines whether IBGP or EBGP neighbors.
 The router bgp command alone does not activate BGP on a router.
 Must enter at least one subcommand.
 Only one instance of BGP can be configured on a router
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Defining BGP Neighbors and Activating
BGP Sessions
RTA(config)#router bgp 100
RTA(config-router)#neighbor 10.1.1.1 remote-as 200
RTB(config)#router bgp 200
RTB(config-router)#neighbor 10.1.1.2 remote-as 100
 neighbor {ip-address | peer-group-name} remote-as autonomous-system
 This address must be reachable (for TCP session) and exchanging BGP
updates.
 autonomous-system field of the neighbor remote-as:
 EBGP: different AS numbers
 IBGP: same AS number
 Note: We will discuss peer groups later.
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router bgp 65101
neighbor 10.2.2.2 remote-as 65101
neighbor 10.1.1.2 remote-as 65101
EBGP and fully meshed IBGP
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Router(config)# router bgp as
Router(config-router)# neighbor {ip-address | peer-group-name} shutdown
 Shutting Down a BGP Neighbor
 To disable (administratively shut down) an existing BGP neighbor or
peer group
 This command not only terminates the session but also removes all
associated routing information.
 Use the “no neighbor…” to enable a neighbor previously shutdown
 If you want to implement major policy changes to a neighboring you must:
 Administratively shut down the neighboring router
 Implement the changes (attributes, route filtering, etc.)
 Administratively bring the neighboring router back up
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Defining the Source IP Address
RTA(config)#router bgp 100
RTA(config-router)#neighbor 10.1.1.1 remote-as 200
I will only accept BGP updates with a source IP address of 10.1.1.1
 The BGP neighbor statement tells the BGP process the destination IP
address of each update packet.
 The source IP address must match the address in the corresponding
neighbor statement on the other router.
 BGP does not accept unsolicited updates.
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RouterD(config)#router bgp 65102
RouterD(config-router)#neighbor 10.3.3.1 remote-as 650102
 Router D has the configuration above.
 If Router A is sending the BGP packets to Router D via Router B
 The source IP address of the packets will be 10.1.1.1
 Router D:
 Does not recognize the sender of the BGP packet (10.1.1.1)
 10.1.1.1 is not configured as a neighbor of Router D.
 The IBGP session between Router A and D will not be established.
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Router(config)#router bgp as
Router(config-router)# neighbor {ip-address
update-source loopback interface-number
RouterA(config)#router bgp 65102
RouterA(config-router)#neighbor 192.168.4.4
RouterA(config-router)#neighbor 192.168.4.4
RouterD(config)#router bgp 65102
RouterD(config-router)#neighbor 192.168.1.1
RouterD(config-router)#neighbor 192.168.1.1
| peer-group-name}
remote-as 650102
update-source loopback0
remote-as 650102
update-source loopback0
 Solution: Establish the IBGP session using a loopback interface when
there are multiple paths between the IBGP neighbors.
 Causes the router to use the address of the specified loopback interface
as the source address for BGP connections to this neighbor.
 Need to make sure loopback address is reachable (TCP session)
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 Router B has Router A as an EBGP neighbor
 Only reachable via directly connected address 172.16.1.1
 Router C has Router D as an EBGP neighbor
 Only reachable via directly connected address 192.168.1.1
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 However, Router B has multiple paths to reach Router C (IBGP neighbor)
 Router B and C: All networks are reachable via IGP EIGRP including
loopback interfaces
 Neighbor relationship between Routers B and C are loopback interfaces
 If one of the links between Router B and C goes down the BGP peering is
not affected.
 Without the loopbacks as source interfaces If one of the links between
Router B and C goes down the BGP neighbor relationship would be lost.
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EBGP Multihop
 Because IGP routing information is not exchanged with external peers:
 The router must point to a directly connected address for external neighbors.
 A loopback interface is never directly connected.
 So, if you want to peer with a loopback interface instead:
 Must add a static route to the loopback pointing to the physical address
of the directly connected network (the next-hop address)
 Must also enable multihop EBGP, with the router configuration command:
neighbor {ip-address | peer-group-name} ebgp-multihop [ttl]
 This command increases the default of one hop for EBGP peers by changing the
default Time to Live (TTL) value of 1 and therefore allowing routes to the EBGP
loopback address.
 By default, the TTL is set to 255 with this command.
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 This command is of value when redundant paths exist between EBGP neighbors.
EBGP Multihop
RTA(config)#router bgp 65102
RTA(config-router)#neighbor 192.168.1.18 remote-as 65101
RTA(config-router)#neighbor 192.168.1.34 remote-as 65101
Alternative but less desirable
 Router A in AS 65102 has two paths to Router B in AS 65101.
 Primary
 Backup
 Router A can use two neighbor statements pointing to 192.168.1.18 and
192.168.1.34 on Router B.
 Disadvantage: Router A will send duplicate (two) BGP updates to Router B
because of the two neighbor statements.
47
EBGP Multihop
EBGP Multihop
 Each router instead:
 Uses its loopback address as the source IP address for its BGP updates
 Points to the loopback address of the other router
 IGP is not used between autonomous systems, so neither router can reach
the loopback of the other router without assistance.
 Assistance: Each router needs to use two static routes:
 A path to reach the loopback address of the other router.
 neighbor ebgp-multihop: Must also be configured to change the default
setting of BGP and inform the BGP process that this neighbor IP address is
more than one hop away (2 hops in this example).
48
Changing the Next-Hop
Attribute
 An internal protocol, such as RIP, EIGRP, or OSPF, always uses the source
IP address of a routing update as the next-hop address for each network
from that update that is placed in the routing table.
 IBGP: The next-hop is carried into IBGP unchanged.
 neighbor {ip-address | peer-group-name} next-hop-self router
configuration command
 Used to force BGP to use the source IP address of the update as the
next hop for each network it advertises to the neighbor
 Rather than using the next-hop AS IP address (default).
49
192.168.15.0
 EBGP: The next-hop is the IP address of the neighbor that sent the update.
 Router A advertises (EBGP) 192.168.15.0 to Router B, with a next hop of 172.16.1.1
 Router C advertises (EBGP) 192.168.15.0 to Router B, with a next hop of 192.168.1.1
 IBGP: The next-hop is carried into IBGP unchanged.
 Router B advertises (IBGP) 192.168.15.0 to Router C, with a next hop of 172.16.1.1
 Router B uses 172.16.1.1 as the next-hop attribute to get to 192.168.15.0 and
 Router C uses 172.16.1.1 as the next-hop attribute to get to 192.168.15.0
 Router C’s next hop to reach 192.168.15.0 is 172.16.1.1 not one of it’s local interfaces
 Very important that Router C knows how to reach the 172.16.1.1:
 IGP
 Static route
 Otherwise, Router C will drop packets destined for 192.168.15.0, because it will not be able
to get to the next-hop address for that network.
 Must either:
 Advertise the next-hop network, the network between AS’s, (172.16.0.0) into the AS
50
 Change the next-hop address to Router B’s local interface
192.168.15.0
 The neighbor next-hop-self
command to change the default
BGP next-hop settings.
 Router B advertises a next hop of
192.168.2.2 (the IP address of its
loopback interface) to its IBGP
neighbor
 set with the neighbor updatesource command
51
Defining the Networks That BGP
Advertises
RTA(config)#router bgp as
RTA(config-router)#network network-number
 Two options available to advertise networks into BGP:
 network command
 Redistributing routes from an IGP into BGP
 Not recommended because any change in IGP routes (link goes
down) might cause a BGP update
 Could result in unstable BGP tables
 If redistribution:
 Only local routes to the AS are redistributed
 Problem if you redistribute routes learned from other AS’s into
the IGP and the redistribute those routes back into BGP
because it could result in a routing loop.
52
RTA(config)#router bgp as
RTA(config-router)# network network-number [mask network-mask] [routemap map-tag]
network network-number [mask network-mask] [route-map maptag]
 Permits BGP to advertise a network if it is present in the IP routing table
 No mask: Classful default mask is assumed.
 Different than IGP network command:
 Determines which networks this router advertises.
 Does not determine which interfaces send/receive routing protocol updates
 Sole purpose of the network command is to notify BGP which networks to
advertise
 If the mask is not specified router announces only the classful network
address
 At least one subnet of the specified major network must be present in the
IP routing table for BGP to announce the classful network as a BGP route.
 If you specify the network-mask:
 An exact match to the network (both address and mask) must exist in the
53
routing table for the network to be advertised.
Auto-summary
RTA(config)#router bgp as
RTA(config-router)# no auto-summary
 With Cisco IOS 12.1(8)T – Default is no auto-summary
 auto-summary – All redistributed subnets are summarized to their
classful boundaries in the BGP table.
 When a subnet is redistributed from an IGP into BGP, only the classful
network route is injected into the BGP table.
54
BGP Neighbor Authentication
 BGP neighbor authentication on a router so that the router authenticates the
source of each routing update packet that it receives
 Exchange of an authenticating keys (sometimes referred to as a passwords)
 To enable MD5 authentication on a TCP connection between two BGP
peers, use the string router configuration command:
neighbor {ip-address | peer-group-name} password
55
BGP Synchronization
I learned about 172.16.0.0 via IBGP from
Router B. I will not advertise 172.16.0.0 to
Router E via EBGP unless I see this network
in my routing table leaned via an IGP (OSPF).
Note: There is not a physical
link b/t A and B
?
OSPF
IBGP
AS 65000 networks
Into OSPF
RTA(config)#router bgp 65500
RTA(config-router)# synchronization
 synchronization - Router configuration command to enable BGP
synchronization
 Router will not advertise routes in BGP until it learns them in an IGP.
 No longer the default as of Cisco IOS 12.2(8)T
 BGP synchronization rule states: A BGP router should not use or
advertise to an external BGP neighbor a route learned by IBGP, unless that
route is directly connected or learned from the IGP.
56
No BGP
Synchronization
I learned about 172.16.0.0 via IBGP from
Router B. I can
will not
advertise
advertise
172.16.0.0
172.16.0.0
to Router
to
Router
E
via EBGP
E via even
EBGP
if this
unless
network
I see in
this
NOT
network
in my
in my routing
routing
table leaned
table leaned
via anvia
IGPan(OSPF).
IGP (OSPF).
?
OSPF
IBGP
AS 65000 networks
Into OSPF
RTA(config)#router bgp 65500
RTA(config-router)# no synchronization
 no synchronization - Router configuration command disables
synchronization
 Default as of Cisco IOS 12.2(8)T
 Best practice is to no longer redistribute BGP networks into the IGP.
 Instead, all routers in the AS should be fully meshed IBGP.
57
Resetting BGP Sessions
So we can focus on other items, please read this section
on your own.
58
Resetting BGP Sessions
 BGP can potentially handle huge volumes of routing information.
 Cisco IOS Software applies changes on only those updates received or
transmitted after the BGP policy configuration change has been performed.
 If the network administrator wants the policy change to be applied on all
routes, he or she must trigger an update to force the router to let all routes
pass through the new filter.
 Outgoing information, the router has to resend its BGP table through the
new filter
 Incoming information, the router needs its neighbor to resend its BGP
table so that it passes through the new filter
 There are three ways to trigger an update:
 Hard reset
 Soft reset
 Route refresh
59
RTA(config)#router bgp 65500
RTA(config-router)# no synchronization
Hard Reset of BGP Sessions
clear ip bgp * or clear ip bgp {neighbor-address}
 Resetting a session is a method of informing the neighbor or neighbors of a
policy change.
 All routes from that session are invalidated and removed from the BGP
table.
 The remote neighbor detects a BGP session down state:
 Also invalidates the received routes.
 After a period of 30 to 60 seconds, the BGP sessions are reestablished
automatically
 The BGP tables are exchanged again, but through the new filters.
 However, resetting the BGP session disrupts packet forwarding.
60
RTA(config)#router bgp 65500
RTA(config-router)# clear ip bgp *
Or
RTA(config-router)# clear ip bgp neighbor-address
 Router A:
 Has eight neighbors
 Each neighbor sends Router A the full Internet table
 assume that is about 32 MB in size
 If the clear ip bgp * command is issued on Router A:
 All eight routers resend their 32 MB table at the same time.
 Router A will need 256 MB of RAM
 Router A will have to process all of this information.
 This will take a considerable number of CPU cycles
 This will further delaying the routing of user data
 clear ip bgp neighbor-address - one neighbor is reset at a time is
less severe
61
RTA# clear ip bgp * soft out
Or
RTA# clear ip bgp neighbor-address soft out
Soft Reset of BGP Sessions Outbound
clear ip bgp {* | neighbor-address} [soft out]
Note: soft keyword is optional
 Causes BGP to do a soft reset for outbound updates.
 Does not reset the BGP session
 Router creates a new update and sends the whole table to the specified
neighbors.
 Does not have any memory overhead.
 This command is highly recommended when you are changing an
outbound policy
 But does not help if you are changing an inbound policy.
62
RTA(config)#router bgp 65500
RTA(config-router)# neighbor {ip-address} soft-reconfiguration inbound
RTA# clear ip bgp {* | neighbor-address} soft in
Soft Reset of BGP Sessions Inbound
 There are two ways to perform an inbound soft reconfiguration:
 Stored routing update information
 Dynamically
Inbound Soft Reset Using Stored Information
 First: Enter the neighbor {ip-address} soft-reconfiguration inbound router
configuration command to inform BGP to save all updates that were learned from
the neighbor specified.
 The BGP router retains an unfiltered table of what that neighbor has sent.
 Next, the inbound policy is changed
 Then, use clear ip bgp {* | neighbor-address} soft in privileged EXEC command
causes the router to use the stored unfiltered table to generate new inbound
updates; the new results are placed in the BGP forwarding database.
 Thus, if you make changes, you do not have to force the other side to resend
everything.
63
Route Refresh: Dynamic Inbound Soft Reset
 provides automatic support for dynamic soft reset of inbound BGP routing table
updates that is not dependent on stored routing table update information.
 This new method requires no preconfiguration
 The clear ip bgp {* | neighbor-address} [soft in | in] privileged EXEC
command is the only command required for this dynamic soft reconfiguration.
 The soft in option generates new inbound updates without resetting the BGP
session, but it can be memory intensive.
 BGP does not allow a router to force another BGP speaker to resend its entire
table.
 If you change the inbound BGP policy and you do not want to complete a hard
reset, use this command to cause the router to perform a soft reconfiguration.
 The soft in option generates new inbound updates without resetting the BGP
session, but it can be memory intensive.
 BGP does not allow a router to force another BGP speaker to resend its entire
table.
 If you change the inbound BGP policy and you do not want to complete a hard
reset, use this command to cause the router to perform a soft reconfiguration.
64
 When a BGP session is reset using soft reconfiguration, the following
commands can be useful for monitoring the BGP routes received, sent, or
filtered:
 show ip bgp neighbors {address} received-routes: displays all
received routes (both accepted and rejected) from the specified neighbor.
 show ip bgp neighbors {address} routes: displays all routes that
are received and accepted from the specified neighbor.; this output is a
subset of the output displayed by the received-routes keyword.
 show ip bgp: displays entries in the BGP table.
 show ip bgp neighbors {address} advertised-routes: displays
all BGP routes that have been advertised to neighbors
65
BGP Configuration Examples
66
Basic BGP
67
IBGP and
EBGP
Router B
router bgp 65000
neighbor 10.1.1.2 remote-as 64520
EBGP peer to Router A
neighbor 192.168.2.2 remote-as 65000 IBGP peer to Router C’s loopback
neighbor 192.168.2.2 update-source loopback0 IBGP-RTC: Use Lo0 as source
neighbor 192.168.2.2 next-hop-self IBGP-RTC: Changes next-hop attribute to its own Lo0
network 172.16.10.0 mask 255.255.255.0 Advertises LAN 172.16.10.0/24 via BGP
network 192.168.1.0
Advertises serial link 192.168.1.0/24 via BGP
network 192.168.3.0
Advertises serial link 192.168.3.0/24 via BGP
no synchronization
May be default if IOS 12.2(8)T or later. Must be fully-meshed IBGP
router eigrp 10
network 192.168.2.1 255.255.255.255
Advertises Lo0 192.168.2.1/32 via EIGRP so
RTC can reach it with it’s neighbor statement.
RTC will advertise 192.168.2.2/32
68
Verifying and Troubleshooting BGP
 You can verify BGP operation using show EXEC commands, including the
following:
 show ip bgp—Displays entries in the BGP topology database (BGP table).
Specify a network number to get more specific information about a
particular network.
 show ip bgp rib-failure—Displays BGP routes that were not installed in the
routing information base (RIB), and the reason that they were not installed.
 show ip bgp neighbors—Displays detailed information about the TCP and
BGP connections to neighbors.
 show ip bgp summary—Displays the status of all BGP connections.
 Use the show ip bgp ? command on a router to see other BGP show
commands.
 debug commands display events as they happen on the router. For BGP,
the debug ip bgp privileged EXEC command has many options, including
the following:
 dampening—BGP dampening
 events—BGP events
 keepalives—BGP keepalives
 updates—BGP updates
69
show ip bgp




show ip bgp - Displays the BGP topology database (the BGP table).
The origin codes are shown at the end of each line.
The status codes are shown at the beginning of each line of output
In this output, most of the rows have an asterisk (*) in the first column.
 Means that the next-hop address is valid.
 The next-hop address is not always the router that is directly connected to this
router. (IBGP)
 A locally originated route has a next hop of 0.0.0.0
70
Please review on your own…
 s: indicates that the specified routes are suppressed
(usually because routes have been summarized and
only the summarized route is being sent).
 d: for dampening, indicates that the route is being
dampened (penalized) for going up and down too often.
 Although the route might be up right now, it is not
advertised until the penalty has expired.
 h: for history, indicates that the route is unavailable and
is probably down; historic information about the route
exists, but a best route does not exist.
 r: for RIB failure, indicates that the route was not
installed in the RIB.
 The reason that the route is not installed can be
displayed using the show ip bgp rib-failure
command, as described in the next section.
 S: for stale, indicates that the route is stale (this is used
in a nonstop forwarding-aware router
71
 A greater-than sign (>) in the second column indicates the best path for a route
selected by BGP
 This route is offered to the IP routing table.
 asterisk (*) - means that the next-hop address is valid.
72
 Third column is either blank or has an i:
 Blank: BGP learned that route from an external peer (EBGP)
 i: BGP learned that route from an internal peer (IBGP) .
73
 The fourth column lists the networks that the router learned.
 The fifth column lists all the next-hop addresses for each route. This next-hop address
column might contain 0.0.0.0, which signifies that this router originated the route.
 The next three columns list three BGP path attributes associated with the path:
 metric (MED)
 local preference
 weight.
74




“Path” header may contain a sequence of autonomous systems in the path.
The first AS listed is the adjacent AS from which this network was learned.
The last AS this network’s originating AS.
If the path column is blank, the route is from the current AS (this AS)
75
 The last column signifies how this route was entered into BGP on the original router
(the origin attribute).
 i: The original router probably used a network command to introduce this network into
BGP.
 e: The original router learned this network from EGP (legacy, not likely).
 ?: The original BGP process cannot absolutely verify this network’s availability,
because it is redistributed from an IGP into the BGP process
76
show ip bgp rib-failure
Displayed routes were not installed because a route(s) with a better administrative
distance already existed in the RIB.
 show ip bgp rib-failure - Displays BGP routes that were not installed in the
RIB (IP routing table) and the reason that they were not installed.
77
show ip bgp summary
 show ip bgp summary – Verifies BGP neighbor relationships and other
information
78
BGP Router ID
Last version of BGP database that was
Injected into the main routing table
Increases in increments when
the BGP table changes
The IP address, used in the neighbor statement,
with which this router is setting up a relationship
BGP
version
Neighbors AS
Number of
BGP
messages
received
from this
neighbor
Number of
BGP
messages
sent to this
neighbor
The number
of BGP
messages
from this
neighbor
waiting to be
processed
The last
version of
the BGP
table sent to
this neighbor
The number of BGP
messages queued and
waiting to be sent to this
neighbor. TCP flow
control prevents a
router from
overwhelming its
neighbor
The amount of
time this
neighbor has
been in the
current BGP
state
(established,
active or idle)
The current
BGP state. If
established,
not shown,
instead a value
is in the
Pfx/Rcd
When the
session is in
the established
state, this
value
represents the
number of
BGP network
entries
received from
this neighbor
79
debug ip bgp updates
After the neighbor adjacency is reestablished, Router A
creates and sends updates to 10.1.0.2.
Update sent about
network 10.1.1.0/24,
with a next hop of
10.1.0.1, which is
Router A’s address
Update sent about
network
10.97.97.0/24, with a
next hop of
172.31.11.4, which is
the address of one
of Router A’s EBGP
neighbors.
Router A later receives
updates from 10.1.0.2.
containing a path to two
networks, 10.1.2.0/24 and
10.1.0.0/24
80
show ip bgp neighbors
 show ip bgp neighbors – Displays information about the BGP
connections to neighbors.
 The BGP state is established, which means that the neighbors have
established a TCP connection and the two peers have agreed to use BGP
to communicate.
81
Regular Expressions
 A regular expression is a pattern to match against an input string.
Character
Description
^
Matches the beginning of the input
string.
$
Matches the end of the input string.
_
Matches a space, comma, left brace, right
brace, the beginning of an input string, or
the ending of an input stream
.
*
Matches any single character
Matches 0 or more single- or multiplecharacter patterns.
82
192.10.2.0/24
AS 300
RouterA
130.1.50.32/30
AS 200
RouterB
12.0.0.0/8
RouterC#show ip bgp
Network
Next Hop
*> 11.0.0.0
0.0.0.0
*> 12.0.0.0
200.200.200.65
*> 192.10.2.0
200.200.200.65
200.200.200.64/30
AS 400
RouterC
11.0.0.0/8
Metric LocPrf Weight Path
0
32768 i
0 300 200 i
0
0 300 i
RouterC# show ip bgp regexp ^300
 Match beginning of input string, AS_PATH, = 300
 Last prepended AS was 300:
 Routes matched: 12.0.0.0 and 192.10.2.0
83
192.10.2.0/24
AS 300
RouterA
130.1.50.32/30
AS 200
RouterB
12.0.0.0/8
RouterC#show ip bgp
Network
Next Hop
*> 11.0.0.0
0.0.0.0
*> 12.0.0.0
200.200.200.65
*> 192.10.2.0
200.200.200.65
200.200.200.64/30
AS 400
RouterC
11.0.0.0/8
Metric LocPrf Weight Path
0
32768 i
0 300 200 i
0
0 300 i
RouterC# show ip bgp regexp ^200
 Match beginning of input string, AS_PATH, = 200
 Last prepended AS was 200:
 Routes matched : none
84
192.10.2.0/24
AS 300
RouterA
130.1.50.32/30
200.200.200.64/30
AS 200
RouterB
12.0.0.0/8
RouterC#show ip bgp
Network
Next Hop
*> 11.0.0.0
0.0.0.0
*> 12.0.0.0
200.200.200.65
*> 192.10.2.0
200.200.200.65
AS 400
RouterC
11.0.0.0/8
Metric LocPrf Weight Path
0
32768 i
0 300 200 i
0
0 300 i
RouterC# show ip bgp regexp 300$
 Match end of input string, AS_PATH, = 300
 Originating AS = 300:
 Routes matched : 192.10.2.0
85
192.10.2.0/24
AS 300
RouterA
130.1.50.32/30
200.200.200.64/30
AS 200
RouterB
12.0.0.0/8
RouterC#show ip bgp
Network
Next Hop
*> 11.0.0.0
0.0.0.0
*> 12.0.0.0
200.200.200.65
*> 192.10.2.0
200.200.200.65
AS 400
RouterC
11.0.0.0/8
Metric LocPrf Weight Path
0
32768 i
0 300 200 i
0
0 300 i
RouterC# show ip bgp regexp 200$
 Match end of input string, AS_PATH, = 200
 Originating AS = 200:
 Routes matched : 12.0.0.0
86
AS 200
AS 100
2.0.0.0
1.0.0.0
AS 1000
AS 400
AS 300
AS 50
10.0.0.0
4.0.0.0
3.0.0.0
5.0.0.0
AS50#show ip bgp
Network
*> 5.0.0.0
*> 1.0.0.0
*> 2.0.0.0
*> 3.0.0.0
*> 4.0.0.0
*> 10.0.0.0
Path
i
100 i
100 200 i
300 i
300 400 i
300 400 1000 I
AS50#show ip bpg regexp 100
 Match input string, AS_PATH, containing 100, including 1000
 Routes matched : 1.0.0.0, 2.0.0.0, 10.0.0.0
87
AS 200
AS 100
2.0.0.0
1.0.0.0
AS 1000
AS 400
AS 300
AS 50
10.0.0.0
4.0.0.0
3.0.0.0
5.0.0.0
AS50#show ip bgp
Network
*> 5.0.0.0
*> 1.0.0.0
*> 2.0.0.0
*> 3.0.0.0
*> 4.0.0.0
*> 10.0.0.0
Path
i
100 i
100 200 i
300 i
300 400 i
300 400 1000 I
AS50#show ip bpg regexp ^100_
 Match beginning of input string, AS_PATH, = 100
 Last prepended AS was 100:
 Routes matched : 1.0.0.0, 2.0.0.0
88
AS 200
AS 100
2.0.0.0
1.0.0.0
AS 1000
AS 400
AS 300
AS 50
10.0.0.0
4.0.0.0
3.0.0.0
5.0.0.0
AS50#show ip bgp
Network
*> 5.0.0.0
*> 1.0.0.0
*> 2.0.0.0
*> 3.0.0.0
*> 4.0.0.0
*> 10.0.0.0
Path
i
100 i
100 200 i
300 i
300 400 i
300 400 1000 I
AS50# show ip bgp regexp _400$
 Match end of input string, AS_PATH, = 400
 Originating AS = 400:
 Routes matched : 4.0.0.0
89
AS 200
AS 100
2.0.0.0
1.0.0.0
AS 1000
AS 400
AS 300
AS 50
10.0.0.0
4.0.0.0
3.0.0.0
5.0.0.0
AS50#show ip bgp
Network
*> 5.0.0.0
*> 1.0.0.0
*> 2.0.0.0
*> 3.0.0.0
*> 4.0.0.0
*> 10.0.0.0
Path
i
100 i
100 200 i
300 i
300 400 i
300 400 1000 I
AS50#show ip bpg regexp _400_
 Match anywhere in input string, AS_PATH, 400
 Routes matched : 4.0.0.0, 10.0.0.0
90
AS 200
AS 100
2.0.0.0
1.0.0.0
AS 1000
AS 400
AS 300
AS 50
10.0.0.0
4.0.0.0
3.0.0.0
5.0.0.0
AS50#show ip bgp
Network
*> 5.0.0.0
*> 1.0.0.0
*> 2.0.0.0
*> 3.0.0.0
*> 4.0.0.0
*> 10.0.0.0
Path
i
100 i
100 200 i
300 i
300 400 i
300 400 1000 I
AS50#show ip bgp regexp ^300$
 Match input string that starts and ends at 300
 Routes that originated from directly connected AS 300 customer
 Routes matched : 3.0.0.0
91
Configuring BGP Attributes
92
The Route Selection Decision Process (ROUTE)











Step 1 Prefer the route with the highest weight. (Recall
that the weight is Cisco-proprietary
and is local to the
WLam
router only.)
Step 2 If multiple routes haveWeight
the same weight, prefer the
route with the highest local preference. (Recall that the
local preference is used within an AS.)
Local Preference
Step 3 If multiple routes have the same local preference,
prefer the route that was originated by the local router.
as path
(network command)
Step 4 If none of the routes were originated by the local
med
router, prefer the route with the
shortest AS-path.
Step 5 If the AS-path length is the same, prefer the
lowest origin code (IGP < EGP < incomplete).
Step 6 If all origin codes are the same, prefer the path
with the lowest MED. (Recall that the MED is exchanged
between autonomous systems.) The MED comparison is
done only if the neighboring AS is the same for all routes
considered, unless the bgp always-compare-med router
configuration command is enabled.
Step 7 If the routes have the same MED, prefer external
paths (EBGP) over internal paths (IBGP).
Step 8 If synchronization is disabled and only internal
paths remain, prefer the path through the closest IGP
neighbor. This means that the router prefers the shortest
internal path within the AS to reach the destination (the
shortest path to the BGP next-hop).
Step 9 For EBGP paths, select the oldest route, to
minimize the effect of routes going up and down
(flapping).
Step 10 Prefer the route with the lowest neighbor BGP
router ID value.
Step 11 If the BGP router IDs are the same, prefer the
route with the lowest neighbor IP address.
1
2
3
5
4
7
6
8
9
10
11
93
Changing the Weight
WLam
Weight
Local Preference
as path
med






Weight attribute influences only the local router.
When there is more than one route to the same destination.
Cisco attribute
Local to the router and it is not propagated in routing updates.
Default is 32768
Higher weight is preferred when there are multiple routes to the same
destination
94
R1
 Routing policy dictates the selection of AS 65030 as the primary way out of
AS 65040 for the traffic destined to any network originated by the AS 65020.
95
R1
AS _65020$
Weight=150
 This route map set-weight is linked to neighbor 10.0.0.1 (R2) as an inbound route
map.
 The match condition is defined by the match as-path path-list-number
 The path-list-number parameter is the number of the AS access-list (10)
 Checks the AS-path attributes of networks to see which are permitted by AS
access list 10.
 as-path AS access-list 10 permits networks whose:
 AS-path attribute ends with 65020; these are networks originating in AS 65020.
96
 set weight 150: The route map sets these networks to a weight of 150
R1
AS _65020$
Weight=150
 Statement 20 does not have any match statements so all remaining
networks are permitted.
 These remaining networks have their weight set to 100, with the set weight
100 command.
 When Router R1 receives updates from 10.0.0.1 (R2):
 It processes them through the set-weight route map
 Sets the weight accordingly as the networks are placed in Router R1’s
BGP table
97
Accepting only a default/Announcing only your locally source routes
(non-transit)
router bgp 24
no synchronization
bgp log-neighbor-changes
network 257.8.8.0
neighbor
neighbor
neighbor
neighbor
neighbor
256.44.111.231
256.44.111.231
256.44.111.231
256.44.111.231
256.44.111.231
remote-as 7777
description To ISP 1
weight 1000
prefix-list DEFAULTONLY in
route-map LOCALONLY out
neighbor
neighbor
neighbor
neighbor
neighbor
306.12.242.161
306.12.242.161
306.12.242.161
306.12.242.161
306.12.242.161
remote-as 8888
description To ISP 2
weight 2000
prefix-list DEFAULTONLY in
route-map LOCALONLY out
ip as-path access-list 99 permit ^$
route-map LOCALONLY permit 10
match as-path 99
ip prefix-list DEFAULTONLY seq 5 permit 0.0.0.0/0
 The local AS is only added
after processing outbound
AS path access lists.
 ^$: Will check for an empty
path if we want to
announce only our locally
sourced routes, then AS
99999 will be added.
98
Higher
weight
preferred
AS 100
(E2)
1000
AS 1
(ISP1)
2000
E1
Can also set the weight in the neighbor statement (for all routes received from
that neighbor)
E1(config)#router bgp 100
E1(config-router)# neighbor
E1(config-router)# neighbor
E1(config-router)# neighbor
E1(config-router)# neighbor
10.1.1.2 weight 2000
10.1.1.6 weight 2000
128.107.1.2 weight 1000
128.107.2.1 weight 1000
 Routes from ISP1 get a weight of 2000
 Routes from E2 and EX get a weight of 1000
 Larger weight preferred
99
Setting Local Preference
WLam
Weight
Local Preference
as path
med
 Local preference is used only within an AS between IBGP speakers to
determine the best path to leave the AS to reach an outside network.
 Default is 100
 Higher values are preferred
 bgp default local-preference value router configuration command changes
the default local preference to the value specified
 All BGP routes that are advertised include this local preference value.
 The value can be set to a number between 0 and 4294967295.
100
 Local preference for all routes on Router B to 500 and on Router A to 200
 All BGP routers in AS 65001 send all traffic destined for the Internet to
Router B, causing its outbound utilization to be much higher and the
utilization out Router A to be reduced to a minimal amount.
 This change is probably not what the network administrator intended.
 We should use route maps to:
 Set only certain networks to have a higher local preference through
Router B
101
 Each network has two paths (< best path)
 All routes have:
 Weight of 0
 Default local preference of 100
 So, BGP uses the shortest AS-path to select the best routes
102
 172.16.0.0: shortest AS-path is via 192.168.28.1, Router X (65002 65003)
 172.24.0.0: shortest AS-path is via 172.20.50.1, Router Y (65005)
 172.30.0.0: shortest AS-path is via 172.20.50.1, Router Y (65005 65004)
103
 A traffic analysis reveals the following:
 The link going through:
 Router B to 172.20.50.1 is heavily used
 Router A to 192.168.28.1 is hardly used at all
 The three largest-volume destination networks on the Internet from AS
65001 are: 172.30.0.0, 172.24.0.0, and 172.16.0.0.
 30% of Internet traffic is going to network 172.24.0.0 (via Router B)
 20% of Internet traffic is going to network 172.30.0.0 (via Router B)
 10% of Internet traffic is going to network 172.16.0.0 (via Router A)
 40%% of Internet traffic is going to other destinations
 You have decided to divert traffic to network 172.30.0.0 and send it out
Router A to the next hop of 192.168.28.1
 Better load balancing
104
Router A



172.30.0.0
LocPref=400
Router A: The route map is linked to neighbor 192.168.28.1 as an inbound route map.
 Receives updates from 192.168.28.1
 Processes them through the local_pref route map
 Sets the local preference accordingly and adds them to the BGP table.
The first route map statement
 Match condition checks all networks to see which are permitted by access list 65.
 Access list 65 permits 172.30.0.0 network
 set local-preference 400 sets these networks to a local preference of 400
The second route map statement does not have any match or set statements.
 Similar to a permit any statement in an access list.
 Because there are no match conditions for the remaining networks, they are all permitted with
their current settings.
105
 So, the local preference for networks 172.16.0.0 and 172.24.0.0 stays set at the default of 100.
172.30.0.0
LocPref=400
WLam
Weight
Local Preference
as path
med
 Router C learns about the new local preference value (400) coming from Router A for
network 172.30.0.0.
 Only change: the new best route to network 172.30.0.0 is via 192.168.28.1
 Local preference = 400; higher than the default local preference of 100 via 172.20.50.1.
 The AS-path through 172.20.50.1 is still shorter than the path through 192.168.28.1, but
AS-path but local preference takes precedence.
106
Setting AS Path
AS Prepending
By default, the preferred way (shortest
path) to reach networks in AS65010 to
reach 65040 is via R1
 It is complicated to influence other autonomous systems to select a
particular path for traffic that is returning to a specific AS (can use MED)
 Virtually impossible to influence another AS to select the desired path based
on the weight and local preference attributes
 These require configuration changes in the neighboring AS
 Need to have the network administrator for that AS(‘s) make the change
 By default, if no BGP path selection tools are configured to influence traffic
flow, BGP uses the shortest AS path, regardless of available bandwidth.
107
 One way that an AS can attempt to influence incoming traffic flow is by
sending out EBGP updates with an extended AS-path attribute for
undesired paths
 Multiple copies of the AS number of the sender
 Known as AS-path prepending
 Makes it less likely that the receiver of the update will select this as best
path.
 To avoid BGP loop prevention mechanisms, no other AS number, except
that of the sending AS, should be prepended to the AS-path attribute.
108
I now prefer the three AS hop via
65020. I also include this
complete AS path in my BGP
updates to AS 65020
AS Path = 65040 65040 65040 65040
 This route map is linked to neighbor 172.16.1.1 as an outbound route map.
 Therefore, as Router R1 sends updates to 172.16.1.1, it processes them
through the set-AS-path route map
 Route map called set-AS-path has only one statement, a permit statement
with a sequence number of 10.
 All updates sent to neighbor 172.16.1.1 are prepended three times (plus the
original AS number) with the AS number of the sender (65040), making that
path less preferable for the returning traffic.
109
Setting the MED
WLam
Weight
Local Preference
as path
med
 MED is used to decide how to enter an AS when multiple paths exist
between two autonomous systems and one AS is trying to influence the
incoming path from the other AS.
 Default MED is 0
110
 To change this value, use the default-metric number router configuration
command.
 To reach networks in AS 65001 all routers in AS 65004 see:
 A MED of 1001 through the next hop of Router A
 A MED of 99 through the next hop of Router B
 If AS 65004 has no overriding policy, all routers in AS 65004 choose to exit
their AS through Router Y to reach the networks in AS 65001
 This could result in suboptimal bandwidth utilization.
111
Setting the
MED using
Route Maps
 Using route maps we want the preferred exit points to be:
 Router A to reach networks 192.168.25.0/24 and 192.168.26.0/24
 Router B to reach network 192.168.24.0/24
 Remember, lower MED is preferred!
112
Router A
Router B
 Router A: Outbound route map named
med_65004 is linked to neighbor
192.168.28.1
 Router B: Outbound route map named
med_65004 is linked to neighbor
172.20.50.1
113
Router A
Router B
 Router A:
 Networks permitted by ACL 66
(192.168.25.0/24 and
192.168.26.0/24)
 Set MED to 100
 All other networks
(192.168.24.0/24)
 Set MED to 200
114
Router A
Router B
 Router B:
 Networks permitted by ACL 66
(192.168.24.0/24)
 Set MED to 100
 All other networks
(192.168.25.0/24 and
192.168.26.0/24)
 Set MED to 200
115
 Router Z BGP table: Indicating the networks learned from AS 65001.
116
L2
PAIX
Customer
Prefixes
Public IP
Address
Bi-lateral
Settlement
Free Peer
Non-transit.
You can get to
our prefixes and
we can get to
your prefixes.
No charge.
Level 3
Transit
Buy transit per Mb/s per
month for routes not know
from non-transit AS’s
EBGP
CENIC (non-transit AS)
Cabrillo
Google
Redistribute
static
MED
AS and
Public IP
Address
LocPref
SVL-Agg1
SAC
EBGP
UCSC
Hurricane Elec
Fully-meshed IBGP
EBGP
AS and
Public IP
Address
SVL
Commodity
Peer
Commodity
Peer
UCLA
LAX-Agg1
LAX
RVR
Comcast
IGP: IS-IS used for next-hop reachability
Public IP
Address
Commodity
Peer
EBGP
Cuesta
Non-transit. You can get to our prefixes and
we can get to your prefixes. No charge.
Internet 2
Research
Peer
NLR
CUDI
Cogent
Research
Peer
Transit
Buy transit per Mb/s
per month for routes
not know from nontransit AS’s
Note: Non-customer prefixes (networks connecting peers) are not included in EBGP updates to peers.
IGP is not redistributed into IS-IS and IS-IS is not redistributed into BGP.
117
Note: This diagram is by no means accurate but only a very simplified representation of an AS.
Peer Groups
Filtering BGP Updates
Additional Attribute Examples
These sections are fairly straight forward. Please read
this material on your own.
118
Peer Groups
 In BGP, many neighbors are often configured with the same update policies
(for example, they have the same filtering applied).
 On a Cisco Systems router, neighbors with the same update policies can be
grouped into peer groups to simplify configuration and, more importantly, to
make updating more efficient and improve performance.
 Peer group – A group of BGP neighbors of the router being configured that
all of the same update policies.
 A neighboring router can only be a part of one peer group.
 When you have many peers, this approach is highly recommended.
 You can use neighbor peer-group-name peer-group or neighbor peergroup command, the neighbor ip-address peer-group peer-group-name
119
Router C





AS 65100 has four routers running IBGP.
All of these IBGP neighbors are peering with each others’ loopback 0 interface
Router C has an outbound distribution list associated with each IBGP neighbor.
the distribute-list command when used for BGP, it is linked to a specific neighbor.
The ISP behind Router C might be announcing private address space to Router C,
and Router C does not want to pass these networks to other routers running BGP in
AS 65100.
 If Router C receives a change from AS 65101:
 It must generate an individual update for each IBGP neighbor and
 Run each update against distribute-list 20.
 If Router C has a large number of IBGP neighbors, the processing power needed
to inform the IBGP neighbors of the changes in AS 65101 could be extensive.
120
 Router C is using a peer group called internal.
 These commands are all linked to peer group internal, which in turn is linked to each
of the IBGP neighbors.
 neighbor remote-as
 neighbor update-source
 neighbor next-hop-self
 neighbor distribute-list 20 out
 If Router C receives a change from AS 65101
 It creates a single update and
 Processes it through distribute-list 20 once.
 The update is replicated for each neighbor that is part of the internal peer group.
 Saves processing time in generating the updates for all IBGP neighbors.
 Peer groups can improve efficiency when processing updates for BGP neighbors that
have a common outbound BGP policy.
121
Filtering BGP Updates
122
 BGP may receive a high number of routing updates.
 To optimize BGP configuration, route filtering may be applied.
 Filter lists, prefix lists, and route maps can be applied to either incoming or outgoing
BGP information, or in any combination.
 Incoming prefix list, filter list, and route map must all permit the routes that are
received from a neighbor before they will be accepted into the BGP table.
 Outgoing routes must pass the outgoing filter list, prefix list, and route map
before they will be transmitted to the neighbor.
 Redistribution from an IGP into BGP, the routes must successfully pass any prefix list
or route map applied to the redistribution process before the route is injected into the
BGP table.
123
BGP Filtering Using Prefix Lists
 prefix-list ANY-8to24-NET is applied to the incoming advertisements from
the BGP neighbor 172.16.1.2
 Permits routes from:
 0.0.0.0/0: any network
 ge 8 le 24: with a mask length from 8 to 24 bits
124
BGP Filtering with Route Maps
 To apply a route map to filter incoming or outgoing BGP routes, use:
neighbor ip address route-map name {in | out}
125
0.0.0.0/0
Weight=100
0.0.0.0/0
Weight=150
100
Matches all routes
 Router A is configured for BGP with two neighbors: 10.2.3.4 and 10.4.5.6
 Both neighbors are configured with the neighbor route-map command to filter the
incoming routing update traffic according to the route-map named filter.
 The route map filter:
 Matches default route AND from AS 65387
 Assigned a weight value of 150
 Matches default route (from all other AS’s)
 Assigned a weight value of 100
 Since a higher weight value is preferred, the link to ISP AS 65387 is preferred
126
Other Attribute Examples (FYI)
127
neighbor weight
command
 Modify WEIGHT E1 so it prefers the longer AS_PATH through ISP1.
 E2 and EX will still prefer the shorter AS_PATH through ISP2 because of
equal WEIGHTs but shorter AS_PATH.
128
 Before modifying WEIGHT.
 With WEIGHTs equal, E1 prefers ISP2
because of shorter AS_PATH
E1#show ip bgp
Network
* 99.0.0.0
*
*>i
Next Hop
10.1.1.2
10.1.1.6
128.107.2.1
Metric LocPrf Weight Path
0
0 1 10 11 12 13 14 i
0
0 1 10 11 12 13 14 i
0
100
0 2 10 14 i
E1#show ip route
B
99.0.0.0/8 [200/0] via 128.107.2.1, 00:39:01
129
Higher
weight
preferred
AS 100
(E2)
1000
AS 1
(ISP1)
2000
E1
E1(config)#router bgp 100
E1(config-router)# neighbor
E1(config-router)# neighbor
E1(config-router)# neighbor
E1(config-router)# neighbor
10.1.1.2 weight 2000
10.1.1.6 weight 2000
128.107.1.2 weight 1000
128.107.2.1 weight 1000
 Routes from ISP1 get a weight of 2000
 Routes from E2 and EX get a weight of 1000
 Larger weight preferred
130
E1#show ip bgp
Network
99.0.0.0
*
*>
* i
*> 128.107.0.0/19
* i
s> 128.107.1.0/24
s> 128.107.2.0/24
* 180.0.0.0
*>
*>i200.0.0.0
Only the BGP routes from E2 have the WEIGHT of 2000.
The BGP routes from ISP1 have a WEIGHT of 1000.
Next Hop
10.1.1.6
10.1.1.2
128.107.2.1
0.0.0.0
128.107.2.1
0.0.0.0
128.107.1.2
10.1.1.6
10.1.1.2
128.107.2.1
Metric LocPrf Weight Path
0
2000 1 10 11 12 13 14 i
0
2000 1 10 11 12 13 14 i
0
100
1000 2 10 14 i
32768 i
0
100
1000 i
0
32768 i
2
32768 i
0
2000 1 i
0
2000 1 i
0
100
1000 2 i
E1#show ip route
B
B
O
C
B
C
C
B
200.0.0.0/24 [200/0] via 128.107.2.1, 00:00:12
99.0.0.0/8 [20/0] via 10.1.1.2, 00:00:12
128.107.0.0/16 is variably subnetted, 3 subnets, 2 masks
128.107.2.0/24 [110/2] via 128.107.1.2, 00:48:24, FastEthernet0/0
128.107.1.0/24 is directly connected, FastEthernet0/0
128.107.0.0/19 [200/0] via 0.0.0.0, 00:00:12, Null0
10.0.0.0/30 is subnetted, 2 subnets
10.1.1.0 is directly connected, Serial0/0
10.1.1.4 is directly connected, Serial0/1
180.0.0.0/16 [20/0] via 10.1.1.2, 00:00:14
131
E2#show ip bgp
BGP table version is 11, local router ID is 192.168.1.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete
Network
* i99.0.0.0
*>
* i128.107.0.0/19
*>
s> 128.107.1.0/24
s> 128.107.2.0/24
*>i180.0.0.0
*> 200.0.0.0
Next Hop
128.107.1.1
192.168.1.2
128.107.1.1
0.0.0.0
128.107.2.2
0.0.0.0
128.107.1.1
192.168.1.2
Metric LocPrf Weight Path
0
100
0 1 10 11 12 13 14 i
0
0 2 10 14 i
0
100
0 i
32768 i
2
32768 ?
0
32768 ?
0
100
0 1 i
0
0 2 i
E2#show ip route
B
99.0.0.0/8 [20/0] via 192.168.1.2, 00:49:40
 WEIGHT is not included in BGP updates.
 E1 prefers longer path (larger WEIGHT) via ISP1 but…
 E2 and EX are unchanged with WEIGHTs remaining equal
preferring shorter AS_PATH.
132
LOCAL
PREFERENCE
Attribute
Local Preference
200
100
 Configuration of WEIGHT removed.
 Multiple routes to the same destination (99.0.0.0), AS 100 Enterprise routers prefer
shorter AS_PATH to ISP2 via E2.
 The weight attribute was local to the router on which it is assigned, and it is not
propagated in routing updates.
 Multiple routes to the same destination (99.0.0.0/8), we want ALL AS 100 Enterprise
routers to prefer ISP1 - not just E1 (WEIGHT).
 The path with the higher preference is preferred (the default value of the local
preference attribute is 100).
133
200
100
E1(config)#router bgp 100
E1(config-router)#bgp default local-preference 200
E2(config)#router bgp 100
E2(config-router)#bgp default local-preference 100
134
E1#show ip bgp
Network
99.0.0.0
*
*>
* i128.107.0.0/19
*>
s> 128.107.1.0/24
s> 128.107.2.0/24
* 180.0.0.0
*>
*>i200.0.0.0
Next Hop
10.1.1.6
10.1.1.2
128.107.2.1
0.0.0.0
0.0.0.0
128.107.1.2
10.1.1.6
10.1.1.2
128.107.2.1
Metric LocPrf Weight Path
0
0 1 10 11 12 13 14 i
0
0 1 10 11 12 13 14 i
0
100
0 i
32768 i
0
32768 i
2
32768 i
0
0 1 i
0
0 1 i
0
100
0 2 i
E1#show ip route
B
B
O
C
B
C
C
B
200.0.0.0/24 [200/0] via 128.107.2.1, 00:01:01
99.0.0.0/8 [20/0] via 10.1.1.2, 00:01:46
128.107.0.0/16 is variably subnetted, 3 subnets, 2 masks
128.107.2.0/24 [110/2] via 128.107.1.2, 00:18:33, FastEthernet0/0
128.107.1.0/24 is directly connected, FastEthernet0/0
128.107.0.0/19 [200/0] via 0.0.0.0, 00:18:34, Null0
10.0.0.0/30 is subnetted, 2 subnets
10.1.1.0 is directly connected, Serial0/0
10.1.1.4 is directly connected, Serial0/1
180.0.0.0/16 [20/0] via 10.1.1.2, 00:18:21
135
E2#show ip bgp
Network
* 99.0.0.0
*>i
*> 128.107.0.0/19
* i
s> 128.107.1.0/24
s> 128.107.2.0/24
*>i180.0.0.0
*> 200.0.0.0
Next Hop
192.168.1.2
128.107.1.1
0.0.0.0
128.107.1.1
128.107.2.2
0.0.0.0
128.107.1.1
192.168.1.2
Metric LocPrf Weight Path
0
0 2 10 14 i
0
200
0 1 10 11 12 13 14 i
32768 i
0
200
0 i
2
32768 ?
0
32768 ?
0
200
0 1 i
0
0 2 i
E2#show ip route
B
B
C
O
B
C
B
200.0.0.0/24 [20/0] via 192.168.1.2, 00:01:46
99.0.0.0/8 [200/0] via 128.107.1.1, 00:01:46
128.107.0.0/16 is variably subnetted, 3 subnets, 2 masks
128.107.2.0/24 is directly connected, FastEthernet0/0
128.107.1.0/24 [110/2] via 128.107.2.2, 00:19:23, FastEthernet0/0
128.107.0.0/19 [200/0] via 0.0.0.0, 00:01:31, Null0
192.168.1.0/30 is subnetted, 1 subnets
192.168.1.0 is directly connected, Serial0/0
180.0.0.0/16 [200/0] via 128.107.1.1, 00:01:47
136
EX#show ip bgp
Network
*>i99.0.0.0
* i128.107.0.0/19
*>i
*>i180.0.0.0
*>i200.0.0.0
Next Hop
128.107.1.1
128.107.2.1
128.107.1.1
128.107.1.1
128.107.2.1
Metric LocPrf Weight Path
0
200
0 1 10 11 12 13 14 i
0
100
0 i
0
100
0 i
0
100
0 1 i
0
100
0 2 i
EX#show ip route
B
B
C
C
B
B
200.0.0.0/24 [200/0] via 128.107.2.1, 00:02:32
99.0.0.0/8 [200/0] via 128.107.1.1, 00:03:18
128.107.0.0/16 is variably subnetted, 3 subnets, 2 masks
128.107.2.0/24 is directly connected, FastEthernet0/1
128.107.1.0/24 is directly connected, FastEthernet0/0
128.107.0.0/19 [200/0] via 128.107.1.1, 00:20:06
180.0.0.0/16 [200/0] via 128.107.1.1, 00:19:51
137
MED (Multi-Exit Discriminator) Attribute
WLam
Weight
Local Preference
as path
med




New Topology
No ISP2
ISP1 prefers path to 128.0.0.0/7 via E1
AS 100 wants ISP1 to prefer path to 128.0.0.0/7 via E2
138
MED (Multi-Exit Discriminator) Attribute
WLam
Weight
Local Preference
as path
med
 The multi-exit discriminator (MED) attribute is a hint to external neighbors about the
preferred path into an AS when there are multiple entry points into the AS.
 A lower MED value is preferred over a higher MED value.
 The default value of the MED attribute is 0.
 Unlike local preference, the MED attribute is exchanged between AS’s, but a MED
attribute that comes into an AS does not leave the AS.
139
ISP1#show ip bgp
Network
*> 99.0.0.0
* 128.107.0.0/19
*>
*> 180.0.0.0
Next Hop
0.0.0.0
10.1.1.5
10.1.1.1
0.0.0.0
Metric LocPrf Weight Path
0
32768 i
0
0 100 i
0
0 100 i
0
32768 i
ISP1#show ip route
C
B
C
C
C
99.0.0.0/8 is directly connected, Loopback99
128.107.0.0/19 is subnetted, 1 subnets
128.107.0.0 [20/0] via 10.1.1.1, 01:01:26
10.0.0.0/30 is subnetted, 2 subnets
10.1.1.0 is directly connected, Serial0/0
10.1.1.4 is directly connected, Serial0/1
180.0.0.0/16 is directly connected, Loopback180
Currently…
140
E1(config)#router bgp 100
E1(config-router)#neighbor 10.1.1.2 route-map SETMEDOUT out
E1(config)#route-map SETMEDOUT permit 10
E1(config-route-map)#set metric 50
E2(config)#router bgp 100
E2(config-router)#neighbor 10.1.1.6 route-map SETMEDOUT out
E2(config)#route-map SETMEDOUT permit 10
E2(config-route-map)#set metric 10
The multi-exit discriminator
(MED) attribute is a hint to
external neighbors about the
preferred path into an AS when
there are multiple entry points
into the AS.
MED 10
MED 50
141
ISP1#show ip bgp
Network
*> 99.0.0.0
* 128.107.0.0/19
*>
*> 180.0.0.0
MED is sometimes referred to as the metric
Next Hop
0.0.0.0
10.1.1.1
10.1.1.5
0.0.0.0
Metric LocPrf Weight Path
0
32768 i
50
0 100 i
10
0 100 i
0
32768 i
ISP1#show ip route
C
B
C
C
C
99.0.0.0/8 is directly connected, Loopback99
128.107.0.0/19 is subnetted, 1 subnets
128.107.0.0 [20/10] via 10.1.1.5, 00:00:12
10.0.0.0/30 is subnetted, 2 subnets
10.1.1.0 is directly connected, Serial0/0
10.1.1.4 is directly connected, Serial0/1
180.0.0.0/16 is directly connected, Loopback180
MED 10
Note: If you want MED attributes from
neighbors in other AS’s to be
compared, you must configure the bgp
subcommand bgp always-comparemed.
MED 50
142
AS_PATH and
Prepending an AS
WLam
Weight
Local Preference
as path
med
With all else equal shortest
AS_PATH is best
10 11 12 13
AS14
10
99.0.0.0/8
 We want to make it look as if the 99.0.0.0/8 in AS 14 network can be
reached via ISP1 and ISP2.
 We will add this network to both routers and prepend some AS
numbers to make it look like it originated in AS 14 from several AS’s
away.
 Sometimes used by ISPs to prepend their own AS number several
times to make a path look less desirable.
143
ISP1(config)#inter loop 99
ISP1(config-if)#ip add 99.0.0.1 255.0.0.0
ISP1(config)#router bgp 1
ISP1(config-router)#network 99.0.0.0
ISP1(config-router)#neighbor 10.1.1.1 route-map set-99 out
ISP1(config-router)#neighbor 10.1.1.5 route-map set-99 out
ISP1(config)#route-map set-99 permit 10
ISP1(config-route-map)#match ip address prefix-list only-99
ISP1(config-route-map)#set as-path prepend 10 11 12 13 14
ISP1(config)#route-map set-99 permit 20
! All other routes sent as normal
ISP1(config)#ip prefix-list only-99 seq 5 permit 99.0.0.0/8
144
ISP2(config)#inter loop 99
ISP2(config-if)#ip add 99.0.0.1 255.0.0.0
ISP2(config)#router bgp 1
ISP2(config-router)#network 99.0.0.0
ISP2(config-router)#neighbor 192.168.1.1 route-map set-99 out
ISP2(config)#route-map set-99 permit 10
ISP2(config-route-map)#match ip address prefix-list only-99
ISP2(config-route-map)#set as-path prepend 10 14
ISP2(config)#route-map set-99 permit 20
!All other routes sent as normal
ISP2(config)#ip prefix-list only-99 seq 5 permit 99.0.0.0/8
145
E1#show ip bgp
E1 does not send this longer path to E2 and EX
becauseMetric
it heard
better
pathPath
from both.
LocPrf
Weight
Network
Next Hop
* 99.0.0.0
10.1.1.2
*
10.1.1.6
*>i
128.107.2.1
* i128.107.0.0/19
128.107.2.1
*>
0.0.0.0
s> 128.107.1.0/24
0.0.0.0
Best
Path
via
E2
s> 128.107.2.0/24
128.107.1.2
*> 180.0.0.0
10.1.1.2
*
10.1.1.6
*>i200.0.0.0
128.107.2.1
0
0
0
0
0
2
0
0
0
100
100
100
0
0
0
0
32768
32768
32768
0
0
0
1
1
2
i
i
i
i
1
1
2
10 11 12 13 14 i
10 11 12 13 14 i
10 14 i
i
i
i
E1#show ip route
B
B
O
C
B
C
C
B
200.0.0.0/24 [200/0] via 128.107.2.1, 00:39:01
99.0.0.0/8 [200/0] via 128.107.2.1, 00:39:01
128.107.0.0/16 is variably subnetted, 3 subnets, 2 masks
128.107.2.0/24 [110/2] via 128.107.1.2, 00:39:18, FastEthernet0/0
128.107.1.0/24 is directly connected, FastEthernet0/0
128.107.0.0/19 [200/0] via 0.0.0.0, 00:39:12, Null0
10.0.0.0/30 is subnetted, 2 subnets
10.1.1.0 is directly connected, Serial0/0
10.1.1.4 is directly connected, Serial0/1
180.0.0.0/16 [20/0] via 10.1.1.2, 00:38:54
146
E2#show ip bgp
Network
*> 99.0.0.0
* i128.107.0.0/19
*>
s> 128.107.1.0/24
s> 128.107.2.0/24
*>i180.0.0.0
*> 200.0.0.0
Next Hop
192.168.1.2
128.107.1.1
0.0.0.0
128.107.2.2
0.0.0.0
128.107.1.1
192.168.1.2
Metric LocPrf Weight Path
0
0 2 10 14 i
0
100
0 i
32768 i
2
32768 ?
0
32768 ?
0
100
0 1 i
0
0 2 i
E2#show ip route
B
B
C
O
B
C
B
200.0.0.0/24 [20/0] via 192.168.1.2, 00:39:30
99.0.0.0/8 [20/0] via 192.168.1.2, 00:39:30
128.107.0.0/16 is variably subnetted, 3 subnets, 2 masks
128.107.2.0/24 is directly connected, FastEthernet0/0
128.107.1.0/24 [110/2] via 128.107.2.2, 00:39:40, FastEthernet0/0
128.107.0.0/19 [200/0] via 0.0.0.0, 00:39:40, Null0
192.168.1.0/30 is subnetted, 1 subnets
192.168.1.0 is directly connected, Serial0/0
180.0.0.0/16 [200/0] via 128.107.1.1, 00:39:26
147
EX#show ip bgp
Network
*>i99.0.0.0
* i128.107.0.0/19
*>i
*>i180.0.0.0
*>i200.0.0.0
Next Hop
128.107.2.1
128.107.2.1
128.107.1.1
128.107.1.1
128.107.2.1
Metric LocPrf Weight Path
0
100
0 2 10 14 i
0
100
0 i
0
100
0 i
0
100
0 1 i
0
100
0 2 i
EX#show ip route
B
B
C
C
B
B
EX#
200.0.0.0/24 [200/0] via 128.107.2.1, 00:18:20
99.0.0.0/8 [200/0] via 128.107.2.1, 00:18:20
128.107.0.0/16 is variably subnetted, 3 subnets, 2 masks
128.107.2.0/24 is directly connected, FastEthernet0/1
128.107.1.0/24 is directly connected, FastEthernet0/0
128.107.0.0/19 [200/0] via 128.107.1.1, 00:18:21
180.0.0.0/16 [200/0] via 128.107.1.1, 00:18:21
148
CIS 185 CCNP ROUTE
Ch. 6 Border Gateway Protocol Solution
for ISP Connectivity – Part 2
Rick Graziani
Cabrillo College
[email protected]
Last Updated: Fall 2010